Apparatus for comparing an input voltage with a threshold voltage

ABSTRACT

An apparatus for comparing an input voltage with a threshold voltage includes: (a) a first current mirror device that includes a first bipolar transistor with a first base and a first collector; the first base and the first collector establish a diode-connected first collector; the input voltage is received at the first current mirror device; (b) a second current mirror device that includes a second bipolar transistor with a second base and a second collector; the second base and the second collector establish a diode-connected second collector; (c) a first impedance coupled in series with the diode-connected first collector and the diode-connected second collector; and (d) a second impedance coupled between ground and the second current mirror device. The first and second current mirror devices are coupled with an output locus at which output signals appear to indicate relative voltage levels of the input and the threshold voltages.

BACKGROUND OF THE INVENTION

The present invention is directed to apparatuses for comparing an inputvoltage with a reference or threshold voltage. Such apparatuses aresometimes referred to as comparators. The present invention ispreferably embodied in a bandgap comparator apparatus that compares aninput voltage with an inherent threshold voltage presented, orestablished by the apparatus.

A bandgap comparator generates a digital output that transitions highwhen an input voltage exceeds an internally generated reference orthreshold voltage. One problem with typical prior art bandgap comparatorapparatuses is that they draw excessive current when the input voltageexceeds the threshold voltage.

There is a need for an apparatus for comparing an input voltage with athreshold voltage, such as a bandgap comparator, that reduces currentdrain without compromising accuracy of comparator thresholds.

SUMMARY OF THE INVENTION

A preferred embodiment of an apparatus for comparing an input voltagewith a threshold voltage includes: (a) a first current mirror devicethat includes a first bipolar transistor with a first base and a firstcollector; the first base and the first collector establish adiode-connected first collector; the input voltage is received at thefirst current mirror device; (b) a second current mirror device thatincludes a second bipolar transistor with a second base and a secondcollector; the second base and the second collector establish adiode-connected second collector; (c) a first impedance coupled inseries with the diode-connected first collector and the diode-connectedsecond collector; and (d) a second impedance coupled between ground andthe second current mirror device. The first current mirror device andthe second current mirror device are further coupled with an outputlocus. Output signals appearing at the output locus indicate relativevoltage levels of the input voltage and the threshold voltage.

It is, therefore, an object of the present invention to provide anapparatus for comparing an input voltage with a threshold voltage, suchas a bandgap comparator, that reduces current drain without compromisingaccuracy of comparator thresholds.

Further objects and features of the present invention will be apparentfrom the following specification and claims when considered inconnection with the accompanying drawings, in which like elements arelabeled using like reference numerals in the various figures,illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical schematic diagram of a representative prior artcomparator apparatus.

FIG. 2 is an electrical schematic diagram of a comparator apparatusconstructed according to the teachings of the present invention.

FIG. 3 is an electrical schematic diagram of a second embodiment of acomparator apparatus constructed according to the teachings of thepresent invention.

FIG. 4 is an electrical schematic diagram of a circuit segment that maybe employed for constructing a third embodiment of a comparatorapparatus according to the teachings of the present invention.

FIG. 5 is an electrical schematic diagram of a circuit segment that maybe employed for constructing a fourth embodiment of a comparatorapparatus according to the teachings of the present invention.

FIG. 6 is an electrical schematic diagram of a fifth embodiment of acomparator apparatus constructed according to the teachings of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an electrical schematic diagram of a representative prior artcomparator apparatus. In FIG. 1, a prior art comparator apparatus 10 isbased upon a Brokaw bandgap cell, as generally described in “A SimpleThree-Terminal IC Bandgap Reference,” by A. Paul Brokaw; IEEE Journal ofSolid-State Circuits; December 1974; pp. 388-393. Comparator apparatus10 includes a first current mirror 12, a second current mirror 14 and athird current mirror 16. First current mirror 12 includes metal oxidesemiconductor (MOS) transistors M₁, M₂; second current mirror 14includes MOS transistors M₃, M₄; and third current mirror 16 includesMOS transistors M₅, M₆. MOS transistor M₁ has a source 20, a gate 22 anda drain 24. MOS transistor M₂ has a source 26, a gate 28 and a drain 30.MOS transistor M₂ has gate 28 coupled with drain 30 to establish the MOStransistor pair M₁, M₂ as current mirror 12.

MOS transistor M₃ has a source 32, a gate 34 and a drain 36. MOStransistor M₄ has a source 38, a gate 40 and a drain 42. MOS transistorM₃ has gate 34 coupled with drain 36 to establish the MOS transistorpair M₃, M₄ as current mirror 14.

MOS transistor M₅ has a source 44, a gate 46 and a drain 48. MOStransistor M₆ has a source 50, a gate 52 and a drain 54. MOS transistorM₅ has gate 46 coupled with drain 48 to establish the MOS transistorpair M₅, M₆ as current mirror 16. Drain 24 is coupled with drain 48.Drain 42 is coupled with drain 54. A supply voltage V_(CC) is providedat a supply locus 56 that is coupled with sources 20, 26, 32, 38. Source44 is coupled with ground 82. Source 50 is coupled with ground 84.

Comparator apparatus 10 also includes a sensing unit 60 and a scalingunit 62. Sensing unit 60 includes a pair of NPN bipolar transistors Q₁,Q₂. Transistor Q₂ has a collector 64, a base 66 and an emitter 68.Transistor Q₁ has a collector 70, a base 72 and an emitter 74. Emitterarea A_(e2) of emitter 68 of transistor Q₂ is larger than emitter areaA_(e1) of emitter 74 of transistor Q₁ by a factor of N. That is:

 A _(e2) =N·A _(e1)  [1]

Bases 66, 72 are coupled together. An input locus 76 is coupled withbases 66, 72. Resistors R₁, R₂ are coupled with input locus 76 and withan input node 78. Resistors R₁, R₂ scale input signals V_(IN) applied toinput node 78 for presentation at input locus 76.

Scaling unit 62 includes a resistor R₃ coupled with emitter 68 and withemitter 74, and a resistor R₄ coupled between emitter 74 and ground 80.

An output locus 86 is coupled with drains 42, 54; an output voltageV_(OUT) appears at output locus 86 when voltage at input locus 76exceeds a predetermined threshold. The threshold determination isinherent in comparator apparatus 10 and is related to the base-emittervoltage V_(be) of transistor Q₁.

The base-emitter voltage of a bipolar transistor V_(be) equals:$\begin{matrix}{V_{be} = {V_{T}{\ln\left( \frac{I_{C}}{A_{e}J_{s}} \right)}}} & \lbrack 2\rbrack\end{matrix}$

Where V_(T) is the thermal voltage,

-   -   I_(C) is the collector current,    -   A_(e) is the emitter area, and    -   J_(S) is the saturation current density.

If one assumes that current mirrors 12, 14, 16 all implement 1:1 currentratios, then a balance point is attainable at which currents throughtransistors M1, M2, M3, M4 are equal. That is, referring to FIG. 1,I _(M1) =I _(M2) =I _(M3) =I _(M4)  [3]

In such a balance point condition, collector currents I_(C1), I_(C2) oftransistors Q₁, Q₂, respectively, are equal. That is, referring to FIG.1,I_(C1)=I_(C2)  [4]

The voltage across resistor R₃ is therefore equal to the differencebetween base-emitter voltages V_(be1), V_(be2) for transistors Q₁, Q₂,respectively. That is,ΔV _(be) =V _(be1) −V _(be2)  [5]

Substituting expression [2] into expression [5]: $\begin{matrix}{{\Delta\quad V_{be}} = {{V_{T}{\ln\left( \frac{I_{C1}}{A_{e1}J_{s}} \right)}} - {V_{T}{\ln\left( \frac{I_{C2}}{A_{e2}J_{s}} \right)}}}} & \lbrack 6\rbrack\end{matrix}$

Combining terms in expression [6], and canceling out the term J_(S):$\begin{matrix}{{\Delta\quad V_{be}} = {V_{T}{\ln\left( \frac{I_{C1}A_{e2}}{I_{C2}A_{e1}} \right)}}} & \lbrack 7\rbrack\end{matrix}$

Recalling that in balance point conditions I_(C1)=I_(C2), expression [4]becomes, $\begin{matrix}{{\Delta\quad V_{be}} = {V_{T}{\ln\left( \frac{A_{e2}}{A_{e1}} \right)}}} & \lbrack 8\rbrack\end{matrix}$

Recalling further that A_(e2)=N·A_(e1), expression [8] becomes,

 ΔV _(be) =V _(T) ln(N)  [9]

The voltage at input locus 76 may be regarded as the bandgap voltageV_(bg) for comparator apparatus 10. If one ignores base currents (forthe sake of simplicity) one may observe that: $\begin{matrix}{V_{bg} = {{2\quad V_{T}\frac{R_{4}}{R_{3}}{\ln(N)}} + V_{be2}}} & \lbrack 10\rbrack\end{matrix}$

The factor (2V_(T)) is necessary because currents from both transistorsQ₁, Q₂ flows through resistor R₄, thus doubling the total voltage seenthere. Resistors R₃, R₄ are derived from matched devices, so anytemperature dependencies they may possess are equal and thereforecancel. Emitter area ratio N is substantially independent oftemperature. The thermal voltage V_(T) equals: $\begin{matrix}{V_{T} = \frac{kT}{q}} & \lbrack 11\rbrack\end{matrix}$

Where k is Boltzmann's constant,

-   -   T is absolute temperature, and    -   q is the charge on an electron.

Since k and q are temperature-invariant fundamental constants, thethermal voltage V_(T) depends linearly upon absolute temperature. Thefirst term on the right side of expression [10] therefore has a linearpositive temperature coefficient. The second term on the right side ofexpression [10] is base-emitter voltage of transistor Q₁. Thebase-emitter voltage of a silicon bipolar transistor has a negativetemperature coefficient of approximately −2 millivolts per degree Kelvin$\left( {- \frac{2m\quad V}{K}} \right).$The base-emitter voltage of a bipolar transistor varies nonlinearly withtemperature. If this relationship is expanded as a power series upontemperature, the linear term will be found to dominate. If resistors R₃,R₄ are properly selected, the positive temperature coefficient of thefirst term of expression [10] will exactly cancel the linearcontribution of the second term of expression [10]. Under theseconditions, bandgap voltage V_(bg) becomes substantially independent oftemperature. If transistors Q₁, Q₂ are fabricated from silicon, then thevalue of bandgap voltage V_(bg) for which the temperature coefficient issmallest typically equals about 1.25 volts, a value that is indirectlyrelated to the bandgap voltage of silicon. It is for this reason thatvoltage V_(bg) is referred to as the bandgap voltage.

Adjustment of a threshold voltage at which the output of comparatorapparatus 10 transition may be effected by appropriate selection ofvalues for resistors R₁, R₂. Thus, a threshold voltage V_(th) at inputnode 78 can be established by the relation: $\begin{matrix}{V_{th} = {\frac{R_{1} + R_{2}}{R_{2}}\left\lbrack {{2\quad V_{T}\frac{R_{4}}{R_{3}}\ln(N)} + V_{be2}} \right\rbrack}} & \lbrack 12\rbrack\end{matrix}$

If one assumes that V_(bg)≡1.25 volts, then $\begin{matrix}{V_{th} \cong {1.25\left( \frac{R_{1} + R_{2}}{R_{2}} \right)}} & \lbrack 13\rbrack\end{matrix}$

That is, comparator apparatus 10 may be configured to switch (i.e., topresent an output signal V_(OUT) at output locus 86) at any thresholdgreater than approximately 1.25 volts by appropriately selecting valuesfor resistors R₁, R₂.

As mentioned briefly earlier herein, a significant problem withcomparator apparatus 10 is that it draws significant current when inputvoltage V_(IN) at input node 78 exceeds threshold voltage V_(th).Comparator apparatus 10 has five circuit limbs, so its minimum quiescentsupply current I_(Q) at the threshold is 5·I_(min), where I_(min) is theminimum current that can be conducted through a path from supply toground without experiencing significant variations due to junctionleakage, thermal noise or other parasitics. I_(Q) increases rapidly athigher input voltages. There is a need for a lower current comparatorapparatus. A good way to realize this goal is to reduce the number ofpaths through which current flows from supply to ground.

FIG. 2 is an electrical schematic diagram of a comparator apparatusconstructed according to the teachings of the present invention. In FIG.2, a comparator apparatus 110 includes a first current mirror 112 and asecond current mirror structure 114. Second current mirror structure 114is coupled with an impedance, preferably a resistor R₂, to establish acurrent generating circuit 162. First current mirror 112 includesbipolar PNP transistors Q₁, Q₂; second current mirror 114 includesbipolar NPN transistors Q₃, Q₄. Bipolar transistor Q₁ has an emitter120, a base 122 and a collector 124. Bipolar transistor Q₂ has anemitter 126, a base 128 and a collector 130. Bipolar transistor Q₃ hasan emitter 164, a base 166 and a collector 168. Bipolar transistor Q₄has an emitter 170, a base 172 and a collector 174. Base 122 of bipolartransistor Q₁ is coupled with collector 124 of transistor Q₁ and base128 of transistor Q₂ to establish first current mirror 112. Base 166 ofbipolar transistor Q₃ is coupled with collector 168 of transistor Q₃ andbase 172 of transistor Q₄ to establish second current mirror structure114. Resistor R₂ is coupled with emitter 170 of transistor Q₄ and toground 180 to establish current generating circuit 162.

Comparator apparatus 110 also includes scaling unit or element 161.Scaling element 161 determines the threshold voltage V_(th) ofcomparator unit 110, as will be described hereinafter. Scaling element161 includes an impedance, preferably a resistor R₁ coupled in seriesbetween collector 124 and collector 168. Emitter area A_(e2) of emitter170 of transistor Q₄ is larger than emitter area A_(e1) of emitter 164of transistor Q₃ by a factor of N. That is, from expression [1]:A _(e2) =N·A _(e1)  [1]

Emitter 164 is coupled with ground 182. Collectors 130, 174 are coupledtogether. An input locus 178 is coupled with emitters 120, 126. Anoutput locus 186 is coupled with collectors 130, 174; an output voltageV_(OUT) appears at output locus 186 when voltage at input locus 178exceeds a predetermined threshold. The threshold determination isinherent in comparator apparatus 110 and is related to the base-emittervoltages V_(be) of transistors Q₁, Q₃.

At threshold voltage V_(th), current mirror 112 ensures that collectorcurrents are equal at collectors 124, 130. In such circumstances, thevoltage across impedance R₂ (from expression [9]) is:ΔV _(be) =V _(T) ln(N)  [9]

As a consequence, current through transistor Q₂ (and, thus, currentthrough transistor Q₁) equals: $\begin{matrix}{I_{C2} = \frac{V_{T}{\ln(N)}}{R_{2}}} & \lbrack 14\rbrack\end{matrix}$

Where I_(C2) is the current through collector 130 of transistor Q₂.

Thus, at the balance point, the threshold voltage V_(th) must equal:$\begin{matrix}{V_{th} = {{V_{T}\frac{R_{1}}{R_{2}}{\ln(N)}} + {2V_{be}}}} & \lbrack 15\rbrack\end{matrix}$

If threshold voltage V_(th) is set to equal twice the bandgap voltageV_(bg), then the temperature dependence of the threshold voltage V_(th)is minimized. In such a configuration, comparator apparatus 110 acts asa bandgap comparator with a threshold of approximately 2.5 volts, afigure that is associated with the semiconductor material used inmanufacturing transistors Q₁, Q₂, Q₃, Q₄. For purposes of thisillustrative example, transistors Q₁, Q₂, Q₃, Q₄ are presumed to havebeen manufactured in silicon. In contrast to comparator apparatus 10(FIG. 1), comparator apparatus 110 has only two circuit limb currents,so its minimum quiescent supply current I_(Q) at the threshold is2·I_(min). Thus, comparator apparatus 110 is a lower current comparatorapparatus than is comparator apparatus 10 (FIG. 1).

Output impedance characteristics of comparator apparatus 110 may beimproved by adding impedances 119, 125 in series with emitters 120, 126,thereby improving accuracy of threshold switching. Impedances 119, 125are indicated in dotted line format to indicate their characterizationas an alternate embodiment.

Comparator apparatus 110 has a fixed threshold voltage V_(th)=2·V_(bg).Providing an impedance 121 connected between base 122 and emitter 120 oftransistor Q₁ allows selection of a threshold voltage V_(th)>2·V_(bg).Adjusting of threshold voltage V_(th) is effected by appropriateselection of the value of the impedance 121. Impedance 121 is indicatedin dotted line format to indicate its characterization as an alternateembodiment.

Comparator apparatus 110 does not provide a definite output signal whenV_(IN) is less than 2·V_(be). This may be corrected by injecting a smallstartup current I_(start) from a current source 165 connected with base166 of transistor Q₃. So long as startup current I_(start) is muchsmaller than minimum current I_(min) in the various current limbs ofcomparator apparatus 110, startup current I_(start) will have littleeffect upon threshold voltage V_(th) of comparator apparatus 110. Withthe addition of current source 165, the output 186 will be asserted forinput voltages V_(IN) as low as one V_(be). Current source 165 can beimplemented as a depletion-mode FET (field effect transistor), apinched-off JFET (junction field effect transistor), or similarstructure. Current source 165 is indicated in dotted line format toindicate its characterization as an alternate embodiment.

FIG. 3 is an electrical schematic diagram of a second embodiment of acomparator apparatus constructed according to the teachings of thepresent invention. In FIG. 3, a comparator apparatus 210 issubstantially similar to comparator apparatus 110 (FIG. 2) withadditional structure that permits establishing higher thresholdvoltages. Comparator apparatus 210 includes a first current mirror 212and a second current mirror structure 214. Second current mirrorstructure 214 is coupled with an impedance, preferably a resistor R₂ toestablish a current generating circuit 262. First current mirror 212includes bipolar PNP transistors Q₁, Q₂; second current mirror structure214 includes bipolar NPN transistors Q₃, Q₄. Bipolar transistor Q₁ hasan emitter 220, a base 222 and a collector 224. Bipolar transistor Q₂has an emitter 226, a base 228 and a collector 230. Bipolar transistorQ₃ has an emitter 264, a base 266 and a collector 268. Bipolartransistor Q₄ has an emitter 270, a base 272 and a collector 274. Base222 of bipolar transistor Q₁ is coupled with collector 224 of transistorQ₁ and base 228 of transistor Q₂ to establish first current mirror 212.Base 266 of bipolar transistor Q₃ is coupled with collector 268 oftransistor Q₃ and base 272 of transistor Q₄ to establish second currentmirror structure 214. Resistor R₂ is coupled with emitter 270 oftransistor Q₄ and with ground 280 to establish current generatingcircuit 262.

Comparator apparatus 210 also includes a scaling unit or element 261.Scaling element 261 determines the threshold voltage V_(th) ofcomparator unit 210. Scaling element 261 includes an impedance,preferably a resistor R₁ coupled in series between collector 224 andcollector 268. Emitter area A_(e2) of emitter 270 of transistor Q₄ islarger than emitter area A_(e1) of emitter 264 of transistor Q₃ by afactor of N. That is:A _(e2) =N·A _(e1)  [16]

Emitter 264 is coupled with ground 282. Collectors 230, 274 are coupledtogether. An input locus 278 is coupled with emitters 220, 226. Anoutput locus 286 is coupled with collectors 230, 274; an output voltageV_(OUT) appears at output locus 286 when voltage at input locus 278exceeds a predetermined threshold. The threshold determination isinherent in comparator apparatus 210 and is related to the base-emittervoltages V_(be) of transistors Q₁, Q₃. Threshold determination is alsofurther related to diode elements 290 a, 290 b, . . . 290 n coupled inseries with impedance R₁. Diode elements 290 n each add a voltage dropsubstantially equal to V_(be) to threshold determinations for comparatorapparatus 210. Diode elements 290 n are illustrated as coupled betweenimpedance R₁ and collector 268. Diode elements 290 n could as well becoupled in series with impedance R₁ between collector 224 and impedanceR₁. Diode elements 290 n could be employed with the alternateembodiments indicated in comparator apparatus 110 (FIG. 2) in dottedline format.

FIG. 4 is an electrical schematic diagram of a circuit segment that maybe employed for constructing a third embodiment of a comparatorapparatus according to the teachings of the present invention. In FIG.4, a circuit segment 300 is illustrated for effecting adjustment ofthreshold voltage V_(th). Circuit segment 300 includes an NPN bipolartransistor 302 having a collector 304, a base 306 and an emitter 308 andconnection terminals 310, 312. Resistors R_(A), R_(B) affect the voltagedrop V_(d) between terminals 310, 312 as: $\begin{matrix}{V_{d} = {\frac{R_{A} + R_{B}}{R_{B}} \cdot V_{be}}} & \lbrack 17\rbrack\end{matrix}$

Circuit segment 300 may be included for providing threshold levelcontrol, for example, in a comparator apparatus such as comparatorapparatus 210 (FIG. 3) using connection terminals 310, 312 to couplecircuit segment 300 in series with resistor R₁. The advantage of circuitsegment 300 is that it allows continuous adjustment of the thresholdvoltage V_(th) rather than adjustment by discrete steps, as is the casewhen using diodes 290 n (FIG. 3). Circuit segment 300 could be employedwith the alternate embodiments indicated in comparator apparatus 110(FIG. 2) in dotted line format.

FIG. 5 is an electrical schematic diagram of a circuit segment that maybe employed for constructing a fourth embodiment of a comparatorapparatus according to the teachings of the present invention. In FIG.5, a circuit segment 400 is illustrated for effecting contribution tothreshold voltage V_(th). Circuit segment 400 includes an NPN bipolartransistor 402 having a collector 404, a base 406 and an emitter 408 andconnection termini 410, 412. Collector 404 and base 406 are coupledtogether to configure collector 404 as a diode-connected collector.Circuit segment 400 may be included for contributing to threshold level,for example, in a comparator apparatus such as comparator apparatus 210(FIG. 3) using connection terminals 410, 412 to couple circuit segment400 in series with in series with impedance R₁ as a substitute for adiode element 290 n (FIG. 3) or in addition to a diode element 290 n.Circuit segment 400 could be employed with the alternate embodimentsindicated in comparator apparatus 110 (FIG. 2) in dotted line format orwith circuit segment 300 (FIG. 4).

FIG. 6 is an electrical schematic diagram of a fifth embodiment of acomparator apparatus constructed according to the teachings of thepresent invention. In FIG. 6, a comparator apparatus 510 issubstantially similar to comparator apparatus 110 (FIG. 2) withadditional structure that provides hysteresis to avoid multipletransitions of the output V_(OUT) during operation of comparatorapparatus 510 when input V_(IN) is near threshold voltage V_(th).Comparator apparatus 510 includes a first current mirror 512 and asecond current mirror structure 514. Second current mirror structure 514is coupled with an impedance, preferably a resistor R₂ to establish acurrent generating circuit 562. First current mirror 512 includesbipolar PNP transistors Q₁, Q₂; second current mirror structure 514includes bipolar NPN transistors Q₃, Q₄. Bipolar transistor Q₁ has anemitter 520, a base 522 and a collector 524. Bipolar transistor Q₂ hasan emitter 526, a base 528 and a collector 530. Bipolar transistor Q₃has an emitter 564, a base 566 and a collector 568. Bipolar transistorQ₄ has an emitter 570, a base 572 and a collector 574. Base 522 ofbipolar transistor Q₁ is coupled with collector 524 of transistor Q₁ andbase 528 of transistor Q₂ to establish first current mirror 512. Base566 of bipolar transistor Q₃ is coupled with collector 568 of transistorQ₃ and base 572 of bipolar transistor Q₄ to establish second currentmirror structure 514. Resistor R₂ is coupled to emitter 570 oftransistor Q₄ and to ground 580 to establish current generating circuit562.

Comparator apparatus 510 also includes a scaling unit or element 561.Scaling element 561 determines the threshold voltage V_(th) ofcomparator unit 510. Scaling unit 561 includes an impedance, preferablya resistor R₁ coupled in series between collector 524 and collector 568.Emitter area A_(e2) of emitter 570 of transistor Q₄ is larger thanemitter area A_(e1) of emitter 564 of transistor Q₃ by a factor of N.That is:A _(e2) =N·A _(e1)  [18]

Emitter 564 is coupled with ground 582. Collectors 530, 574 are coupledtogether. An input locus 578 is coupled with emitters 520, 526. Anoutput locus 586 is coupled with collectors 530, 574; an output voltageV_(OUT) appears at output locus 586 when voltage at input locus 578exceeds a predetermined threshold. The threshold determination isinherent in comparator apparatus 510 and is related to the base-emittervoltages V_(be) of transistors Q₁, Q₃.

A hysteresis circuit 600 includes a digital buffer 612 for providing anindication of output voltage V_(OUT) to a MOS transistor 604. Digitalbuffer 612 could be constructed, for example, by a series connection oftwo CMOS (complementary metal oxide semiconductor) inverters. MOStransistor 604 includes a drain 606, a gate 608 and a source 610. Drain606 is coupled in common with resistor R₁ and a second impedance,preferably a resistor R₃. Source 610 is coupled with connection 567 thatcommonly couples base 566 with collector 568 of bipolar transistor Q₁.When output 586 is low, hysteresis circuit 600 operates to raise thethreshold voltage V_(th). Because output 586 is low, transistor 604 doesnot conduct and resistor R₃ is placed in series with resistor R₁. This,in turn, increases threshold voltage V_(th), as represented byexpression [15]. When output 586 is high, transistor 604 conducts,effectively shorting resistor R₃ and reducing threshold voltage V_(th).The amount of hysteresis V_(hys) provided by hysteresis circuit 600equals: $\begin{matrix}{V_{hys} = \frac{{R_{3} \cdot V_{t}}{\ln(N)}}{R_{2}}} & \lbrack 19\rbrack\end{matrix}$

Therefore, hysteresis circuit 600 can be adjusted to provide anarbitrarily small hysteresis. A small hysteresis is desirable because itis not possible to adjust both thresholds of apparatus 510 (FIG. 6) tozero temperature coefficients. So long as the hysteresis is less than orequal to about 50 mV, this shortcoming has little impact upon theoperation of apparatus 510. Hysteresis circuit 600 could be employedwith the alternate embodiments indicated in comparator apparatus 110(FIG. 2) in dotted line format, or with circuit segment 300 (FIG. 4) orwith circuit segment 400 (FIG. 5).

It is to be understood that, while the detailed drawings and specificexamples given describe preferred embodiments of the invention, they arefor the purpose of illustration only, that the apparatus of theinvention is not limited to the precise details and conditions disclosedand that various changes may be made therein without departing from thespirit of the invention which is defined by the following claims:

1. A comparator for effecting comparison of an input voltage with athreshold voltage; the comparator comprising: (a) a first current mirrordevice; said first current mirror device including a first bipolartransister, having a first base and a first collector, said first baseand said first collector being connected to a diode; said input voltagebeing received at said first current mirror device; (b) a second currentmirror device; said second current mirror device including a secondbipolar transistor having a second base and a second collector, saidsecond base and said second collector establishing a diode; (c) a firstimpedance coupled in series with said diode being connected to saidfirst collector and said diode being connected to said second collector;and (d) a second impedance coupled between ground and said secondcurrent mirror device; said first current mirror device and said secondcurrent mirror device being further coupled with an output locus; outputsignals appearing at said output locus indicating comparative voltagelevels of said input voltage and said threshold voltage.
 2. Thecomparator for effecting comparison of an input voltage with a thresholdvoltage as recited in claim 1 wherein said first current mirror devicefurther includes a third bipolar transistor having a third collector,and wherein said second current mirror device further including a fourthbipolar transistor having a fourth collector and an emitter; said thirdcollector and said fourth collector being coupled with the output locus;said second impedance being coupled with said emitter of said fourthbipolar transistor.
 3. The comparator for effecting comparison of aninput voltage with a threshold voltage as recited in claim 1 whereinsaid first bipolar transistor presents a first voltage drop and saidsecond bipolar transistor presents a second voltage drop, said thresholdvoltage being established as a function of said first voltage drop andsaid second voltage drop.
 4. An apparatus for effecting comparison of aninput voltage with a threshold voltage; the apparatus comprising: (a) acurrent mirror; said current mirror including a first bipolar transistorhaving a first base, a first emitter, and a first collector; and asecond bipolar transistor having a second base, a second emitter and asecond collector; said first base being coupled to said first collectorand said second base; said input voltage being received at said firstemitter and said second emitter; (b) a current generating circuit; saidcurrent generating circuit including a third bipolar transistor having athird base, a third emitter and a third collector, a fourth bipolartransistor having a fourth base, a fourth emitter and a fourthcollector; and a first resistor; said third base being coupled with saidthird collector and said fourth base; said first resistor being coupledbetween said fourth emitter and a common voltage potential; said thirdemitter being coupled with said common voltage potential; and (c) ascaling element coupled between said first collector and said thirdcollector; said second collector being coupled with said fourthcollector and with an output node; output signals appearing at saidoutput node indicating relative voltage levels of said input voltage andsaid threshold voltage, wherein said third fourth collector and saidsecond collector being coupled with an output locus; and wherein saidsecond bipolar transistor and said fourth bipolar transistor are ofunequal size; one bipolar transistor of said second bipolar transistorand said fourth bipolar transistor being larger than the other bipolartransistor of said second bipolar transistor and said fourth bipolartransistor by a factor of N.
 5. An apparatus for effecting comparison ofan input voltage with a threshold voltage as recited in claim 4 whereinthe apparatus has a configuration for substantially consistentoperational response over a range in temperature.
 6. An apparatus foreffecting comparison of an input voltage with a threshold voltage asrecited in claim 4 wherein said threshold voltage is renderedsubstantially independent of temperature by selection of said firstresistor, said scaling element and said factor of N.
 7. An apparatus foreffecting comparison of an input voltage with a threshold voltage; theapparatus comprising: (a) a current mirror; said current mirrorincluding a first bipolar transistor having a first base, a firstemitter, and a first collector; and a second bipolar transistor having asecond base, a second emitter and a second collector; said first basebeing coupled to said first collector and said second base; said inputvoltage being received at said first emitter and said second emitter;(b) a current generating circuit; said current generating circuitincluding a third bipolar transistor having a third base, a thirdemitter and a third collector; a fourth bipolar transistor having afourth base, a fourth emitter and a fourth collector; and a firstresistor; said third base being coupled with said third collector andsaid fourth base; said first resistor being coupled between said fourthemitter and a common voltage potential; said third emitter being coupledwith said common voltage potential; and (c) a scaling element coupledbetween said first collector and said third collector; said secondcollector being coupled with said fourth collector and with an outputnode; output signals appearing at said output node indicating relativevoltage levels of said input voltage and said threshold voltage, whereinsaid current mirror further includes a fifth bipolar transistor having afifth collector, and wherein said fourth collector and said secondcollector being coupled with an output locus; and wherein said secondbipolar transistor and said fourth bipolar transistor are of unequalsize; one bipolar transistor of said second bipolar transistor and saidfourth bipolar transistor being larger than the other bipolar transistorof said second bipolar transistor and said fourth bipolar transistor bya factor of N.
 8. An apparatus for effecting comparison of an inputvoltage with a threshold voltage as recited in claim 7 wherein theapparatus has a configuration for substantially consistent operationalresponse over a range in temperature.
 9. A comparator for effectingcomparison of an input voltage with a threshold voltage; the comparatorcomprising: (a) a first current mirror device; said first current mirrordevice including a first bipolar transistor having a first base and afirst collector, said first base and said first collector establishing adiode-connected first collector; said input voltage being received atsaid first current mirror device; (b) a second current mirror device;said second current mirror device including a second bipolar transistorhaving a second base and a second collector, said second base and saidsecond collector establishing a diode-connected second collector; (c) afirst impedance coupled in series with said diode connected firstcollector and said diode connected second collector; and (d) secondimpedance coupled between ground and said second current mirror device;said first current mirror device and said second current mirror devicebeing further coupled with an output locus; output signals appearing atsaid output locus indicating comparative voltage levels of said inputvoltage and said threshold voltage, wherein said first bipolartransistor presents a first voltage drop and said second bipolartransistor presents a second voltage drop, and wherein the comparatorfurther comprises at least one additional device in series with saidfirst impedance; each respective device of said at least one additionaldevice presenting a respective additional voltage drop; said thresholdvoltage being established as a function of said first voltage drop, saidsecond voltage drop and said at least one said respective additionalvoltage drop, and wherein the comparator has a configuration forsubstantially consistent operational response over a range intemperature; said configuration being effected by a combination ofinteger N, said first impedance and said second impedance.
 10. Thecomparator for effecting comparison of an input voltage with a thresholdvoltage as recited in claim 9 wherein the second current mirror deviceincludes a third bipolar transistor, said second bipolar transistor andsaid third bipolar transistor are of unequal size; one bipolartransistor of said second bipolar transistor and said third bipolartransistor being larger than the other bipolar transistor of said secondbipolar transistor and said third bipolar transistor by a factor of N.11. An apparatus for effecting comparison of an input voltage with athreshold voltage; the apparatus comprising: (a) a first current mirrordevice; said first current mirror device including a first bipolartransistor having a first base and a first collector, said first baseand said first collector establishing a diode-connected first collector;said input voltage being received at said first current mirror device;(b) a second current mirror device; said second current mirror deviceincluding a second bipolar transistor having a second base and a secondcollector, said second base and said second collector establishing adiode-connected second collector; (c) a first impedance coupled inseries with said diode connected first collector and said diodeconnected second collector; and (d) a second impedance coupled betweenground and said second current mirror device; said first current mirrordevice and said second current mirror device being further coupled withan output locus; output signals appearing at said output locusindicating relative voltage levels of said input voltage and saidthreshold voltage, wherein the apparatus further comprises a thirdimpedance; said third impedance being coupled in series with said firstimpedance between said first current minor device and said secondcurrent mirror device; said third impedance including a switching devicecoupled with said output locus; said switching device switchablybypassing said third impedance in response to said output signals. 12.An apparatus for effecting comparison of an input voltage with aninherent threshold voltage as recited in claim 11 wherein said firstbipolar transistor presents a first voltage drop and said second bipolartransistor presents a second voltage drop, and wherein the apparatusfurther comprises at least one additional device in series with saidfirst impedance; each respective device of said at least one additionaldevice presenting a respective additional voltage drop; said thresholdvoltage being established as a function of said first voltage drop, saidsecond voltage drop and said at least one said respective additionalvoltage drop.
 13. An apparatus for effecting comparison of an inputvoltage with an inherent threshold voltage as recited in claim 12wherein the second current mirror device includes a third bipolartransistor, said third bipolar transistor occupies a larger area thansaid second bipolar transistor by a factor of N; the apparatus having aconfiguration for substantially consistent operational response over arange in temperature; said configuration being effected by a combinationof N, said first impedance and said second impedance.
 14. An apparatusfor effecting comparison of an input voltage with a threshold voltage;the apparatus comprising: (d) a first current mirror device; said firstcurrent mirror device including a first bipolar transistor having afirst base and first collector, said first base and said first collectorestablishing a diode-connected first collector; said input voltage beingreceived at said first current mirror device; (e) a second currentmirror device; said second current mirror device including a secondbipolar transistor having a second base and a second collector, saidsecond base and said second collector establishing a diode-connectedsecond collector; (f) a first impedance coupled in series with saiddiode connected first collector and said diode connected secondcollector; and (d) second impedance coupled between ground and saidsecond current mirror device; said current mirror device and said secondcurrent mirror device being further coupled with an output locus; outputsignals appearing at said output locus indicating relative voltagelevels of said input voltage and said threshold voltage; wherein saidfirst bipolar transistor presents a first voltage drop and said secondbipolar transistor presents a second voltage drop, and wherein theapparatus further comprises at least one additional device in serieswith said first impedance; each respective device of said at least oneadditional device presenting a respective additional voltage drop; saidthreshold voltage being established as a function of said first voltagedrop, said second voltage drop and said at least one said respectiveadditional voltage drop, wherein the apparatus has a configuration forsubstantially consistent operational response over a range intemperature; said configuration being effected by a combination ofinteger N, said first impedance and said second impedance, wherein thesecond current mirror device includes a third bipolar transistor, saidsecond bipolar transistor and said third bipolar transistor are ofunequal size; one bipolar transistor of said second bipolar transistorand said third bipolar transistor being larger than the other bipolartransistor of said second bipolar transistor and said third bipolartransistor by a factor of N, and wherein the apparatus further comprisesa third impedance; said third impedance being coupled in series withsaid first impedance between said first current mirror device and saidsecond current mirror device; said third impedance including a switchingdevice coupled with said output locus; said switching device switchablybypassing said third impedance in response to said output signals.
 15. Acomparator for effecting comparison of an input voltage with a thresholdvoltage; the comparator comprising: (a) a first current mirror device;said first current mirror device including a first bipolar transistorhaving a first base and a first collector, said first base and saidfirst collector being connected to a diode; said input voltage beingreceived at said first current mirror device; (b) a second currentmirror device; said second current mirror device including a secondbipolar transistor having a second base and a second collector, saidsecond base and said second collector establishing a diode-connectedsecond collector; (c) a first impedance coupled in series with saiddiode connected first collector and said diode connected secondcollector; and (d) a second impedance coupled between ground and saidsecond current mirror device; said first current mirror device and saidsecond current mirror device being further coupled with an output locus;output signals appearing at said output locus indicating relativecomparative voltage levels of said input voltage and said thresholdvoltage, wherein said comparator includes a scaling element being thefirst impedance coupled in series with the diode.
 16. An apparatus foreffecting comparison of an input voltage with a threshold voltage; theapparatus comprising: (a) a current mirror; said current mirrorincluding a first bipolar transistor having a first base, a firstemitter, and a first collector; and a second bipolar transistor having asecond base, a second emitter and a second collector; said first basebeing coupled to said first collector and said second base; said inputvoltage being received at said first emitter and said second emitter;(b) a current generating circuit; said current generating circuitincluding a third bipolar transistor having a third base, a thirdemitter and a third collector; a fourth bipolar transistor having afourth base, a fourth emitter and a fourth collector; and a firstresistor; said third base being coupled with said third collector andsaid fourth base; said first resistor being coupled between said fourthemitter and a common voltage potential; said third emitter being coupledwith said common voltage potential, and (c) a scaling element coupledbetween said first collector and said third collector; said secondcollector being coupled with said fourth collector and with an outputnode; output signals appearing at said output node indicating relativevoltage levels or said input voltage and said threshold voltage, whereinsaid fourth bipolar transistor has a first emitter area and said thirdbipolar transistor has a second emitter area; said first emitter areabeing greater than said second emitter area by a factor of N, whereinsaid scaling element is coupled with said output node; said scalingelement being variable; said scaling element responding to voltagepresent at said output node for adjusting said threshold voltage.